On Wed, May 08, 2024 at 08:41:29PM -0700, Roman Gushchin wrote: > Cgroups v2 have been around for a while and many users have fully adopted them, > so they never use cgroups v1 features and functionality. Yet they have to "pay" > for the cgroup v1 support anyway: > 1) the kernel binary contains useless cgroup v1 code, > 2) some common structures like task_struct and mem_cgroup have never used > cgroup v1-specific members, > 3) some code paths have additional checks which are not needed. > > Cgroup v1's memory controller has a number of features that are not supported > by cgroup v2 and their implementation is pretty much self contained. > Most notably, these features are: soft limit reclaim, oom handling in userspace, > complicated event notification system, charge migration. > > Cgroup v1-specific code in memcontrol.c is close to 4k lines in size and it's > intervened with generic and cgroup v2-specific code. It's a burden on > developers and maintainers. > > This patchset aims to solve these problems by: > 1) moving cgroup v1-specific memcg code to the new mm/memcontrol-v1.c file, > 2) putting definitions shared by memcontrol.c and memcontrol-v1.c into the > mm/internal.h header > 3) introducing the CONFIG_MEMCG_V1 config option, turned on by default > 4) making memcontrol-v1.c to compile only if CONFIG_MEMCG_V1 is set > 5) putting unused struct memory_cgroup and task_struct members under > CONFIG_MEMCG_V1 as well. > > This is an RFC version, which is not 100% polished yet, so but it would be great > to discuss and agree on the overall approach. > > Some open questions, opinions are appreciated: > 1) I consider renaming non-static functions in memcontrol-v1.c to have > mem_cgroup_v1_ prefix. Is this a good idea? > 2) Do we want to extend it beyond the memory controller? Should > 3) Is it better to use a new include/linux/memcontrol-v1.h instead of > mm/internal.h? Or mm/memcontrol-v1.h. > Hi Roman, A very timely and important topic and we should definitely talk about it during LSFMM as well. I have been thinking about this problem for quite sometime and I am getting more and more convinced that we should aim to completely deprecate memcg-v1. More specifically: 1. What are the memcg-v1 features which have no alternative in memcg-v2 and are blocker for memcg-v1 users? (setting aside the cgroup v2 structual restrictions) 2. What are unused memcg-v1 features which we should start deprecating? IMO we should systematically start deprecating memcg-v1 features and start unblocking the users stuck on memcg-v1. Now regarding the proposal in this series, I think it can be a first step but should not give an impression that we are done. The only concern I have is the potential of "out of sight, out of mind" situation with this change but if we keep the momentum of deprecation of memcg-v1 it should be fine. I have CCed Greg and David from Google to get their opinion on what memcg-v1 features are blocker for their memcg-v2 migration and if they have concern in deprecation of memcg-v1 features. Anyone else still on memcg-v1, please do provide your input. thanks, Shakeel
